We can currently add black space holders, it would be nice if there were image holders for the dashboard as well.
You can do this with the "Custom HTML" resource. Just add it to you page and use the html img tag
<img src="img.jpg" height="50" width="50">
You can play around with the height and width to gain the desired effect.
I concur... it's a little strange doing it like on a webpage but it does work! :^}
as mentioned by Terrance, this is already doable via the custom HTML resource
